This invention relates to power control in cellular telecommunication systems and, more particularly, to a method and system for power control on a common channel that may be shared by a plurality of mobile stations operating in a cellular telecommunication system.
Power control for mobile station transmissions is an important aspect of cellular telecommunication systems. One type of cellular technology in which power control is particularly important is code division multiple access (CDMA) technology. In a CDMA-type system multiple users, each using a channel identified by a uniquely assigned digital code, simultaneously communicate with the system while sharing the same wideband frequency spectrum. In a CDMA cellular system, the signals from multiple mobile stations are received at the same frequency simultaneously at a base station. Because of the nature of CDMA demodulation, it is necessary that the signal received at the base station from each mobile station be as close as possible to a single power level so that the signal from one mobile station does not overwhelm the signal from another mobile station (near-far problem).
In a CDMA cellular system a power control process may be used to control each mobile station""s transmission power level so that the signal level received at the base station from each mobile station is as close as possible to a single predetermined level. Additionally, the power control process may also be used to assure that the received signal levels at the base station are of an adequate level, so that calls are not dropped. One example of a CDMA mobile station power control scheme is the power control used by systems specified in the
Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) IS-95 standard. Another related CDMA mobile station power control scheme is the power control used by systems specified in the ANSI-008 standard, which is the personal communications system (PCS) 1900 MHz version of IS-95.
In IS-95, a mobile station first adjusts its transmission power level using an access channel assigned to a base station through which the mobile station is attempting to gain access to the system. To gain access, the mobile station follows an open loop power control process that involves transmitting access probe transmissions at a relatively low power level on the access channel and gradually increasing the level of subsequent access probe transmissions in access probe correction increments set by the system, until a response is obtained from the system and the mobile station gains access to the system. The access probe includes an access preamble and an access channel message capsule. The transmission power of each access probe transmission on the access channel is given by the equation:                                                                         Mean                ⁢                                  xe2x80x83                                ⁢                output                ⁢                                  xe2x80x83                                ⁢                power                ⁢                                  xe2x80x83                                ⁢                                  (                  dBm                  )                                            =                              xe2x80x83                            ⁢                                                -                  mean                                ⁢                                  xe2x80x83                                ⁢                input                ⁢                                  xe2x80x83                                ⁢                power                ⁢                                  xe2x80x83                                ⁢                                  (                  dBm                  )                                                                                                                        xe2x80x83                            ⁢                              -                73                                                                                                        xe2x80x83                            ⁢                                                +                  NOM_PWR                                ⁢                                  xe2x80x83                                ⁢                                  (                  dBm                  )                                                                                                                        xe2x80x83                            ⁢                                                +                  INIT_PWR                                ⁢                                  xe2x80x83                                ⁢                                  (                  dBm                  )                                                                                                                        xe2x80x83                            ⁢                                                +                                      xe2x80x83                                    ⁢                  the                                ⁢                                  xe2x80x83                                ⁢                sum                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                all                ⁢                                  xe2x80x83                                ⁢                access                            ⁢                              xe2x80x83                                                                                                        xe2x80x83                            ⁢                              probe                ⁢                                  xe2x80x83                                ⁢                corrections                                                                        (        1        )            
The values NOM_PWR and INIT_PWR are system parameters having values assigned by the system. The mean input power is the total received power at the mobile station antenna connector.
The ANSI-008 standard equation is similar but, because of the frequency difference, has a constant equal to 76 instead of 73 and also includes an additional value that is added to increase the range of NOM_PWR, the additional value being defined as 16*NOM_PWR_EXT, which is a system parameter. IS-95 could also be modfied to include a similar value.
Once the mobile station gains access to the system, it waits in an idle mode until a call is initiated from either the mobile station to the base station, or from the base station to the mobile station. A dedicated reverse traffic channel and a dedicated forward traffic channel are then assigned for the call. When transmitting on the IS-95 reverse traffic channel, the mobile station initializes at the power level at which access was obtained on the access channel using the open loop process defined by equation (1). Once the reverse traffic channel transmission power level is initialized and the call begins, the system and mobile station then also begin a closed loop power control process. The closed loop power control process allows the signal level received at the base station from each mobile station transmitting on a reverse traffic channel to be set as close as possible to a single predetermined level. In the closed loop power control process, the base station transmits closed loop power control corrections in the form of power control bits to the mobile station that are transmitted in a power control subchannel that is included in the forward traffic channel. A single power control bit is transmitted in the power control subchannel every 1.25 msec. A xe2x80x9conexe2x80x9d bit transmitted in the power control subchannel indicates that the mobile station should increase its transmission power 1 dB, while a xe2x80x9czeroxe2x80x9d bit indicates that the mobile station should decrease its transmission power 1 dB. Each time a valid control bit is received at the mobile station in the power control subchannel, the mobile station adjusts its output power level up or down in an increment of 1 dB. The mobile station is capable of adjusting the transmission power within a range of xc2x124 dB around the level set by the open loop power control process on the access channel and, as the call is ongoing, the transmission power of the mobile station on the reverse traffic channel is adjusted so that a desired power level is reached and maintained. The mobile station also simultaneously continues the open loop power control process, this time using the forward traffic channel. In the open loop process the mean input power received on the forward traffic channel is the determining value. When involved in the call, the inputs that effect changes in the mobile station""s transmission power are the mean input power as received from the base station on the forward traffic channel and the closed loop power corrections indicated by the power control bits received on the forward power control channel. During a call, the mobile station output power level on the reverse traffic channel is given by the equation:                                                                         Mean                ⁢                                  xe2x80x83                                ⁢                output                ⁢                                  xe2x80x83                                ⁢                power                ⁢                                  xe2x80x83                                ⁢                                  (                  dBm                  )                                            =                              xe2x80x83                            ⁢                                                -                  mean                                ⁢                                  xe2x80x83                                ⁢                input                ⁢                                  xe2x80x83                                ⁢                power                ⁢                                  xe2x80x83                                ⁢                                  (                  dBm                  )                                                                                                                        xe2x80x83                            ⁢                              -                73                                                                                                        xe2x80x83                            ⁢                                                +                  NOM_PWR                                ⁢                                  xe2x80x83                                ⁢                                  (                  dBm                  )                                                                                                                        xe2x80x83                            ⁢                                                +                  INIT_PWR                                ⁢                                  xe2x80x83                                ⁢                                  (                  dBm                  )                                                                                                                        xe2x80x83                            ⁢                                                +                                      xe2x80x83                                    ⁢                  the                                ⁢                                  xe2x80x83                                ⁢                sum                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                all                ⁢                                  xe2x80x83                                ⁢                access                            ⁢                              xe2x80x83                                                                                                        xe2x80x83                            ⁢                              probe                ⁢                                  xe2x80x83                                ⁢                corrections                                                                                                        xe2x80x83                            ⁢                                                +                                      xe2x80x83                                    ⁢                  the                                ⁢                                  xe2x80x83                                ⁢                sum                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                all                ⁢                                  xe2x80x83                                ⁢                closed                ⁢                                  xe2x80x83                                ⁢                loop                                                                                                        xe2x80x83                            ⁢                              power                ⁢                                  xe2x80x83                                ⁢                control                ⁢                                  xe2x80x83                                ⁢                corrections                                                                        (        2        )            
As for equation (1), the ANSI-008 standard equation is similar but, because of the frequency difference, has a constant equal to 76 instead of 73 and also includes an additional value that is added to increase the range of NOM_PWR, the additional value defined as 16*NOM_PWR_EXT, which is a system parameter.
A third generation CDMA system is being developed to provide more sophisticated and better services (data, etc.) than provided by IS-95 and eventually to replace IS-95. In the proposed standard for third generation CDMA known as CDMA 2000 TTU-R RTT, it has been proposed that third generation systems have a common control channel that is not used in IS-95. The reverse common control channel (R-CCCH) has been proposed as a common random access channel to be shared by multiple mobile stations to allow transmissions to the system without explicit authorization by a base station. It is planned that the R-CCCH be used for control signaling and data transmissions. Each R-CCCH is distinguished by a different long code. Each R-CCCH also has a corresponding forward common control channel (F-CCCH) that carries transmissions from a base station. The F-CCCH is shared by multiple mobile stations with each data or control signaling transmission on the F-CCCH including address information indicating the particular mobile station to which it is addressed. Data transmitted on the R-CCCH would be included in the access channel message capsule portion of the access probe. In the CDMA 2000 TTU-R RTT standard, it is proposed that the same type of access probe sequence power control as used for the access channel in IS-95 be used for transmission on the R-CCCH.
Because of differences between IS-95 and third-generation systems, the use of IS-95 access channel or similar type power control on the R-CCCH could cause problems in third generation CDMA systems. If the access probe sequence type is begun and received at too high a transmission power, the base station will not signal the mobile station to adjust its transmission power downward during the transmission and the R-CCCH transmission may disturb transmissions of other mobile stations. Also, the F-CCCH is shared by multiple mobile stations, and transmissions on the R-CCCH may be made in one or more bursts, with each burst including the same or a varying number of frames. With the third generation system R-CCCH potentially carrying a substantial amount of data and signaling, capacity could be lost due to delay from the need to transmit an overly long access probe sequence in order to receive acknowledgment, when the access probe sequence is begun at too low a power level.
It is an object and advantage of this invention to provide a method and system for power control on a channel that is shared by multiple users in a telecommunication system.
Another object and advantage of this invention is to provide a method and system for power control on a channel that is shared by multiple users through random access in a telecommunication system.
A further object and advantage of this invention is to provide a method and system for power control on a channel that carries transmissions in one or more bursts, with each burst including the same or a varying number of frames of varying sizes.
The present invention provides a method and apparatus for power control on a channel that is shared by multiple mobile stations transmitting to base stations in a telecommunication system. The mobile stations may share the channel in the random access mode. The method and apparatus allows the system to set an initial mobile station transmission power level and may be efficiently applied on a channel that carries a transmission in one or more bursts, with each burst including the same or a varying number of frames of varying sizes. The method and apparatus utilizes a common channel that is shared by multiple mobile stations for transmitting the power control signaling from the base station to the mobile station.
In an embodiment of the invention, the method and system is implemented on a reverse common control channel (R-CCCH) of a code division multiple access (CDMA) system. Prior to the transmission of data or control information by a mobile on the R-CCCH, a parameter measurement is performed at the base station on a reverse measurement channel that is associated with the R-CCCH and has a transmission power level that has a predetermined relationship with the transmission power level of the R-CCCH. The reverse measurement channel may be a reverse pilot channel associated with the R-CCCH, and the measurements may be performed during an initial access probe from the mobile station on the R-CCCH. A power value associated with the parameter measured on the measurement channel is then determined. The power value may indicate the ratio of the energy per bit, Eb to the effective noise power spectral density, Nt, value for both the R-CCCH and measurement channel combined. The power value is compared to a predetermined threshold power value in the base station. In the embodiment, the predetermined threshold power value that is compared to the power value may be determined based on a probability of frame error rate that is associated with a desired probability of a transmission on the R-CCCH being successful. The predetermined threshold power value may indicate a threshold ratio of energy per bit, Eb to the effective noise power spectral density, Nt, for both the R-CCCH and measurement channel combined. The base station then transmits a power control message to the mobile station that includes a power control command indicating a power control increment or, alternatively, a transmission power level for the mobile station on the R-CCCH according to the comparison of the power value and the predetermined threshold power value. The power control message may be transmitted to the mobile station on a forward common control channel (F-CCCH). After receiving the power control message, the mobile station transmits the data or control information using an initial transmission power according to the power control command. During the transmission on the R-CCCH, multiple measurements are made on the reverse measurement channel. The time interval Tm between the measurements may be based upon the sizes of the frames available on the F-CCCH for sending the power control messages. The F-CCCH may have multiple frame sizes available. In the embodiment, F-CCCH frame sizes of 5 msec, 10 msec or 20 msec are used, giving a Tm of 5 msec, 10 msec or 20 msec and a power control update frequency of 200 Hz, 100 Hz or 50 Hz. Each power control message may include the electronic serial number (ESN) of the mobile station to which the base station is transmitting the power control message. If a burst of N frames transmitted by the mobile station after receiving the power control message, including the power command, results in a frame error rate greater than the rate used to determine the threshold power value used to generate the power control command, the transmission of the burst may be repeated using a new threshold power level that is determined from a frame error rate that is assigned with a successively higher probability of a burst on the R-CCCH being successful, until the burst is received correctly or a timeout occurs.